Spark-over protective devices for low voltage electrical apparatus



Dec. 29, 1959 HAMIL ETAL 2,919,371

SPARK- PROTECT V EVICES F 0 OW l V0 E ELECTRIC APPARATU Filed April 18, 1955 2 Sheets-Sheet l `F|G- INENTpRs mw De@ 29, 1959 A..HAM|LToN ET AL i2,919,371

SPARK-OVER PROTECTIVE DEVICES FOR LOW VOLTAGE ELECTRICAL APPARATUS 2 Sheets-Sheet 2 Filed April 18, 1955 FIG?.

INVENT'ORS ATTORNEYS United States Patent O vSPARK-GVER.'PROTECTIVE DEVICES FOR LOW VOLTAGE ELECTRICAL APPARATUS Andrew Hamilton, Timperley, and Arthur Naylor, Sal-` ford, England, assignors to Metropolitan-Vickers Electrical Company Limited, London, England, a British company Application April 1s, 195s, serial No. 502,091 YClaims priority, application Great Britain April 2.2, 1954 4 Claims. (ci. 313-292) Silicon carbide electrical resistance material not only i has a non-linear electricalresistance characteristic but also has a low surface spark-over potential which renders it particularly suitable when used in combination with metallic electrodes to form a protective spark gap for low voltage circuits. One way of constructing such gaps is to clamp metal plates or strips against the surface of a block of silicon carbide with parallel edges of the plates `or strips spaced apart the requisite distance to provide the spark gap.

In some `cases metal foil may be used in place of the i plates or strips.

Another method of manufacture consists in spraying the surface of asilicon carbon block with a suitable metallising coating, and during the spraying masking a strip on the surface, as as to form a spark gap between the edges of the metallised surfaces when the mask is removed.

t wil be appreciated that when circuits which operate at very low voltages have to be protected against surges of comparatively small magnitude, not only has the gap width across the surface of the non-linear resistor to be small, but it must be of accurate dimension if the actual brealedown voltage is to fall within reasonable tolerance of the surge voltage for which protection is to be provided. For example, for the protection of telephone circuits which normally operate at 50 to 130 volts, a gap of approximately I@ inch on the surface of a high impedance silicon carbide resistor can be used to provide against surges of 500 volts or over.

Whilst the methods of manufacture above referred to are capable of providing the necessary degree of accuracy required, they are slow and costly and are quite unsuited to the mass production of small sizes of such protective devices on an economic basis.

The main object of the invention is to provide an improved process of manufacturing such devices.

The present invention comprises a method of manufacturing low voltage spark-over gap devices consisting in shaping a resistor body of silicon carbide and a binder in such a manner that the resistor has one or more ridges protruding from the surface, sintering the shaped resistor to produce a hard ceramic-like material, metallising the surface of the resistor and finally machining the ridge or ridges from the summitor summits to form the spark gap edges between the metallic coating on the opposite slopes of each ridge spaced the required distance-apart.

Preferably the shaping process is effected by extruding the mixture of material under pressure as, for example, by using a hollow die with a central mandrel.

2,919,371 Patented Dec. 29, 1959 The ridges preferably have a width which increases progressively from the outer summit to the base. For example, they may be of triangular section and it will be appreciated that the apex of each ridge is machined off and then a sufficient depth of the ridge is machined away until the gap between the edges of the metallic coating on the two slopes is the requisite length. Clearly the greater the depth of the machining the' greater will be the gap length.

Alternatively, of course, the ridges may have a constant width, i.e. they may be of rectangular section. In such a case it will, of course, be necessary to ensure that the width is accurately dimensioned as it willnot be affected by the depth of the machining.

According to another aspect, the invention comprises a low voltage spark-over protective device comprising a resistor body of sintered silicon carbide having a metallic coating, said resistor body'being formed with one or more'.

ridges protruding from the surface, kwhich ridges have a width which increases progressively from the peripheral summit to the root and are machined at the summit so as to leave a spark gap of requisite width between the edges of the remaining metallic coating on the two sides of the ridge.

The silicon carbide body is conveniently formed on opposite sides with recesses for engagement by electrical v contacts` and in such a case at least one gap would be formed across the intervening surfaces between the contacts.

4In order that the invention may be more clearly understood reference will now be made to the accompanying Y drawings, in which:

Fig. 1 is a View in elevation of a device prior to the machining operation;

Fig. 2 is a plan view (Le. showing the ends) of the device shown in Fig. l;

Fig. 3 is a similar view to Fig. 2 but showing the finished device after the machining operation has been carried out; whilst Figs. 4, 5 and 6 respectively show modified arrangements.

Figs. 7 and 8 show two forms of holder for the device, and

Fig. 9 shows the complete device assembled in a holder.

Referring rst to Figs. l and 2, the device comprises a non-linear silicon carbide resistor block 1 having a coating of a conducting metal or alloy (copper'or brass) 2, and having ridges 3 and grooves 4 formed in its surface. As shown, there are two grooves 4 on opposite sides and these are intended for spring contacts. Similarly, in each of the opposite surfaces between the spring contact grooves 4 there is a ridge 3.

Fig. 3 shows the device after the summits of the ridges have been machined away, as by grinding, so as to produce metal spark gap edges 5 separated by at intervening surfaces 6 of non-linear resistance material along the entire length of theresistor. At the same time the ends of the resistor would be free of the metal coating either by grinding or preferably masking them during the spraying operation. As above mentioned the initial stage preferably comprises a process of extrusion under pressure using a central mandrel which leaves a central aperture 7.

Fig. 4 shows a modified arrangement in which there are four ridges 3' there being two ridges on each side of the device.

The top half of Fig. 4 shows the device prior to a machining operation and the bottom half shows it after the machining operation where it will be observed that there are two gaps in series in each path between the contacts.

Fig. 5 shows a further arrangement which is similar to that shown in Figs. 1-3 excepting that the ridges instead of being of triangular cross section as in the previous arrangements are of semi-circular section.

Fig. 6 shows `a still further arrangement in which the ridges are of rectangular section.

Fig. 7 shows one form of holder on which brackets 8 are mounted on a base plate 9 and carry spring arms of, for example, Phosphor bronze 10 at the ends of which are copper contacts 11 which are intended to engage the grooves 4 of the resistor block.

Terminals 12 are provided for external supply leads.

Fig. 8 shows a modified arrangement in which the cantilever spring arms 10 are replaced by double spring arms 13 having copper contacts 11 at each end.

Fig. 9 shows a block of the type shown in Fig. 3 located on a holder of the type shown in Fig. 7.

What We claim as new and desire to secure by Letters Patent of the United States is:

1. A low voltage spark-over protective device comprising a resistor fody of sintered silicon carbide having a metallic coating, at least one ridge protruding from the surface of said body, said ridge having a width which increases progressively from the peripheral summit to the root, said summit comprising a machined surface forming a spark gap of requisite width between the edges of the metallic coating protruding from the machined surface.

2. A low voltage spark-over protective device comprising a resistor body of sintered silicon carbide having a metallic coating, at least one nidge protruding from the surface of said body, said ridge having a width which increases progressively from the peripheral summit to the root, said summit comprising a machined surface forming a spark gap of requisite width between the edges of 'the metallic coating protruding from the machined surface and recesses in said resistor body on each side of the spark gap.

3. A low voltage spark-over protective device cornprising a resistor body of sintered silicon carbide having a metallic coating, said resistor body being of substantially rectangular cross section and formed with ridges extending longitudinally along a pair of opposite faces, said ridges protruding from said faces and having widths which increase progressively from the peripheral summit to the root and machined surfaces to the summits of said ridges providing spark gaps of requisite width between the edges of the metallic coating on the two sides of the ridge and longitudinal grooves in the intervening faces of said resistor body.

4. A low voltage spark-over protective device cornprising a resistor body of sintered silicon carbide having a metallic coating, said resistor body being of substantially rectangular cross section and formed with ridges extending longitudinally along a pair of opposite faces, said ridges protruding from said faces and having widths which increase progressively from the peripheral surnmit to the root and machined surfaces to the summits of said ridges providing spark gaps of requisite width between the edges of the metallic coating on thc two sides of `the ridge and longitudinal grooves in the intervening faces of said resistor body and contacts engaging said longitudinal grooves and adapted both to support the resistor body and to provide electrical connections.

References Cited in the iile of this patent UNITED STATES PATENTS 1,126,975 Furber Feb. 2, 1915 1,152,982 Sherbondy Sept. 7, 1915 1,699,011 Murphy Ian. 15, 1929 1,721,116 Henry July 16, 1929 2,487,437 Goldstein et al. Nov. 8, 1949 2,615,145 Rydbeck Oct. 21, 1952 2,629,066 Eitel Feb. 17, 1953 FOREIGN PATENTS 616,683 Great Britain Ian. 25, 1949 

